The present invention relates to an image sensor for converting optical information into an electrical signal. More particularly, the invention concerns an improved structure of the image sensor which can be fabricated in a miniature size with the overall length of optical path being shortened.
In general, information processing apparatus such as, for example, facsimile or the like is equiped with an information or data input device which includes an image sensor for converting optical information into an electrical signal. In the hitherto known image sensor, the optical path realized in the sensor is necessarily of a great length in order to fulfill the requirement for attaining a desired accuracy, which in turns imposes restriction on miniaturization of the information apparatus as a whole even if endevour is made to increase the integration density of electric circuits as incorporated. Under the circumstance, difficulty is encountered in meeting the existing demand for miniaturization of the information apparatus or system particularly for home use.
FIG. 1 of the accompanying drawings shows a typical one of the conventional facsimile equipment in which a document 4 is irradiated or illuminated with light emitted by a light source 2 wherein light reflected by the document 4 is focused onto a photodiode 8 through an optical lens system 6. In this case, the size of image projected onto the photodiode 8 is about one tenth (1/10) of that of the document, while the distance from the document to the photodiode, i.e. the length of the optical path is about 300 to 500 mm. Consequently miniaturization of the apparatus is difficult because of great length of the optical path.
FIG. 2 of the accompanying drawings shows another example of the conventional image sensor destined for use in the facsimile equipment. Referring to the figure, a document 4 is illuminated with light emitted by light sources 2, the resulting reflected light being directed onto a photodiode array 12 by way of a gradient-index fiber array 10. In the case of this image sensor, the image projected on the photodiode array 12 is of a same size as that of the image on the document, while the optical path length can be reduced down to 10 to 20 mm as compared with the arrangement shown in FIG. 1. Accordingly, the apparatus can be considerably reduced in the overall size as compared with the apparatus shown in FIG. 1. However, when the facsimile equipment is to be capable of handling the document of a large size such as A4 format, the number of fibers which constitute the gradient-index fiber array 10 amounts to as many as about one thousand, involving complicated structure as well as high manufacturing cost.
FIG. 3 shows a further example of the already proposed image sensor. Referring to the figure, a transparent substrate 20 has a major surface deposited with a light shielding film 22 over which transparent insulation films or layers 24 and 26 are formed, wherein a photosensor array 28 is disposed within the insulation layers 24 and 26. In the case of this image sensor, the document 4 is positioned in intimate contact with the insulation layer 26. Light emitted by a light source 2 is introduced from the rear side of the transparent substrate 20 and illuminates the document 4 through the transparent substrate 20, windows formed in the light shielding film 22 and the transparent insulation films 24 and 26. The light reflected by the document is projected onto the photosensor array 28 through the transparent insulation film 26. It will be seen that the reflected light is projected onto the photosensor array without using a lens in the case of this image sensor, which thus provides advantages in that the optical path length can be reduced to 0.1 to 0.2 mm by virtue of the intimate contact between the document and the insulation film or layer 26 and that the lens is rendered unnecessary, making thus it possible to realize the sensor in a simplified structure. On the other hand, however, since the transparent insulation layer or film 26 which is destined to protect the photosensors 28 is brought into contact with the document, the surface of the insulation film 26 which contacts slideably with the document 4 may be abrased and becomes opaque, to a disadvantage.
The structures shown in FIGS. 2 and 3 are disclosed in "4-th Topical Meeting on Gradient-Index Optical Imaging Systems", July 4-5, 1983, p.p. 220-223 and 236-239.
Furthermore, there is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 105668/83 an image sensor of a structure in which a transparent insulation substrate 39 and a document 4 are brought into slideable contact with each other, wherein a groove 32 of an arcuate cross-section is formed in the slideably contacting surface of the transparent insulation substrate 39 at the location where light is reflected from the document 4, as shown in FIG. 4 of the accompanying drawings. With this structure, the portion of the transparent insulation substrate 39 through which the optical path extend is protected from becoming opaque. However, since light reflected from the document 4 undergoes scattering at the groove 32, the amount of light falling on the photosensor 34 is decreased, resulting in a corresponding lower output of the photosensor, to a disadvantage.